1. Technical Field of the Invention
The present invention relates generally to the field of integrated circuits and, more particularly, to a method and system of selecting a unique combination of materials and dimensions for fabricating a micro-electromechanical switch device.
2. Description of Related Art
Rapid advances made in the field of telecommunications have been paced by improvements in the electronic devices and systems which make the transfer of information possible. Switches which allow the routing of electronic signals are important components in any communication system. Electrical switches are widely used in microwave circuits for many communication applications such as impedance matching, adjustable gain amplifiers, and signal routing and transmission. Current technology generally relies on solid state switches such as PIN diodes.
PIN diodes are typically fabricated in Si and GaAs. Often these devices are combined in series or shunt configurations to produce multipole-multithrow devices. Series configurations are used when minimum insertion loss is required over a broad frequency range. The shunt configuration is used when high isolation is required over a broad frequency range. This design also has better power handling. Multipole-multithrow switch can be fabricated with combinations of series and shunt diodes to take advantage of the benefits of each configuration. A figure of merit for the performance of a switch is given by the cutoff frequency:
f=1/(2 xcfx80Ron Coff),xe2x80x83xe2x80x83Equation 1
where Coff is the capacitance of the device when no voltage is applied and Ron is the resistance of the device with an applied voltage. Though the PIN diode is a popular RF switch, this device typically exhibits high power consumption and degradation at high frequencies. Furthermore, at high frequencies the figure of merit of the PIN switch decreases and results in an increased insertion loss and decreased isolation.
The technology of micro-machining offers a means of improving the increased insertion loss and decreased isolation at high frequencies as an alternative to PIN semiconductor electronic components. New structures, such as micro-machined electromechanical (MEM) switches, allow the design and functionality of integrated circuits to expand in a new dimension, creating an emerging technology with applications in a broad spectrum of technical fields.
An excellent example of a MEM switch is the drumhead capacitive switch structure which is fully described in U.S. Pat. No. 5,619,061. In brief, an input RF signal comes into the structure through one of the electrodes (bottom electrode or membrane electrode) and is transmitted to the other electrode when the membrane is in contact with the dielectric covering the bottom electrode.
The figure of merit for the above-described MEM switch is several times greater than Si PIN switches and is better than GaAs PIN switches. This is because the xe2x80x9conxe2x80x9d resistance of the MEM switch is substantially due to the conductor resistance and is not dependent on contact and channel resistance of a PIN device resulting in low insertion loss of the MEM device. Further, the ratio of the xe2x80x9conxe2x80x9d to xe2x80x9coffxe2x80x9d capacitance (Con/Coff) determines how isolated the device is between poles in multipole devices. This can be tuned to the requirements of the application.
One of the limitations of this device is the switching speed. The RF MEM Switch switching speed is slow when compared to its PIN diode counterpart because of its relatively large inertia and the mechanical properties of the film. The pull-down voltage (Vp) or operating voltage is also larger for similar reasons. The challenge of making these devices is choosing the correct materials and the right dimensions to maximize the device performance for the particular application of interest. Uniquely fabricated MEM switches with improved switching speed offer an alternative to switching RF signals with low power consumption, low insertion loss and good isolation.
The present invention achieves technical advantages as an apparatus and method of selecting a unique combination of materials and dimensions for fabrication of a micro-electromechanical switch for improved RF switch performance. An electrode material is selected which exhibits a resistivity resulting in improved insertion loss performance for a predetermined switching speed requirement of the micro-electromechanical switch. A dielectric material is also selected which exhibits a permittivity resulting in improved isolation performance for the predetermined switching speed in which isolation is a function of the permittivity. Next, an airgap distance from a membrane electrode to a bottom electrode/dielectric surface is selected which results in a pull-down voltage approximately equal to a supply voltage of the micro-electromechanical switch in which the isolation, pull-down voltage, and switching speed are also all functions of the airgap thickness.
RF micro-electromechanical switches of the present invention are low power and only require power during the switching transient, are integrateable into current Si-CMOS technologies and exhibit little or no intermodulation distortion. Additionally, since the process for making RF micro-electromechanical switches only requires seven masks levels, it is more economical than the current GaAs switches.